Microfluidic pumping and dispensing of liquid chemical reagents is the subject of three U.S. Pat. Nos. 5,585,069, 5,593,838, and 5,603,351, all assigned to the David Sarnoff Research Center, Inc. The system uses an array of micron sized reservoirs, with connecting microchannels and reaction cells etched into a substrate. Electrokinetic pumps comprising electrically activated electrodes within the capillary microchannels proved the propulsive forces to move the liquid reagents within the system. The electrokinetic pump, which is also known as an electroosmotic pump, has been disclosed by Dasgupta et al., see "Electroosmosis: A Reliable Fluid Propulsion System for Flow Injection Analyses", Anal. Chem. 66, pp 1792-1798 (1994). The chemical reagent solutions are pumped from a reservoir, mixed in controlled amounts, and then pumped into a bottom array of reaction cells. The array may be decoupled from the assembly and removed for incubation or analysis.
The above described microfluidic pumping device can be used as a display. The pumped fluids to be displayed become colored solutions comprising colorants such as dyes or pigments. The array of reaction cells may be considered display chambers to be used for picture elements, or pixels, in a display, comprising mixtures of pigments having the hue of the pixel in the original scene. Such a display has the advantage that it may be changed simply by pumping new fluids to the display chambers. However, such a display has stability problems. Liquids may evaporate, plugging the apparatus. Moreover, liquids are mobile and may mix together, thus spoiling the accurate display of the hues of the original scene. When changing the display there is the problem of the disposal of spent fluids. It would be desirable to have a microfluidic pumped display that had a stable image that could be easily changed.